Case Studies

Future fuel
The increasing global demand to reduce greenhouse gas and CO2 emissions is driving the need to commercialise renewable energy technologies. Fuel cells provide energy that has high electrical efficiency and is more efficient than conventional combustion-based technologies. They use widely available natural gas and renewable fuels and have low emissions, so offer a promising alternative to fossil fuels.

Fuel cell technology has been around since the 1950s and has been used in many specialist applications that can afford the high costs, for example space missions. Engineers are now striving to develop affordable fuel cell technologies that can be used in other applications, for example to replace batteries in personal devices such as mobile phones and laptops, to power vehicles and for power in the home and industry.

Research and development to reduce the cost of fuel cells and make them commercially viable is being undertaken by a number of international organisations, including Siemens and GE Energy. Morgan Technical Ceramics is working with many of the key players to provide material and manufacturing expertise for the test assemblies, which are a key component to enable the development of more cost efficient solid oxide fuel cells.

The design of more efficient solid oxide fuel cells is expected to provide a practical, alternative source of energy on a large scale for applications including industrial power generation.

This article outlines various fuel cell technologies available and the importance of testing, to provide valuable feedback for development. It also discusses the essential mechanical and electrical characteristics and properties of ceramic test assemblies, housings and fixtures, to ensure accurate test results.

How fuel cells work
Fuel cells produce electricity by a simple electrochemical reaction between fuel (on the anode side) and an oxidant (on the cathode side). The two different gas plenas are separated by an ionic conductive electrolyte and a catalyst, such as platinum or nickel, is often used to speed up the reactions at the electrodes.

The many different types of fuel cells are classified according to the electrolyte. Each has a different operating temperature with associated advantages and disadvantages, making them suitable for various applications. The most common types of fuel cells are proton exchange membrane (PEM), Alkaline (AFC), phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide (SOFC).

Proton Exchange Membrane Fuel Cells (PEM)
PEM fuel cells use a polymer membrane and have low operating temperatures (50-100°C). They are compact and work at high efficiencies of between 40-50 per cent.
However, as a result of operating at a low temperature, they need to use a highly sensitive catalyst to speed up the reactions. This means the cells need pure hydrogen to operate because the catalysts are extremely susceptible to poisoning by carbon monoxide. As a result they are ideal for use in automotive applications and small domestic applications, such as replacements for rechargeable batteries. Work is being carried out to produce more tolerant catalyst systems along with membranes capable of operating at higher temperatures.

Alkaline Fuel Cells (AFC)
Alkaline fuel cells are one of the most developed technologies and are one of the cheapest types of fuel cells to manufacture. The design is similar to that of a PEM cell but it has a stabilized matrix of potassium hydroxide or an aqueous solution as the electrolyte. Alkaline cells work at a similar temperature to PEM cells and start quickly, but their power density is lower. This means they are too bulky for use in car engines but are ideal in small stationary power generation units. The catalysts are extremely sensitive to carbon monoxide and other impurities and the input gases have to be free from carbon dioxide as this reacts with the potassium hydroxide electrolyte forming potassium carbonate, which slows down the cell performance.

Phosphoric Acid Fuel Cells (PAFC)
PAFCs use liquid phosphoric acid as the electrolyte, which is usually contained in a silicone carbide matrix. They work at slightly higher temperatures of between 150 to 200°C, but still require platinum catalysts on the electrodes to speed up reactivity. This increased temperature gives a higher tolerance to impurities, and PAFCs can function with up to two per cent carbon monoxide and a few parts per million of sulphur in the reactant streams. Their efficiency is lower than that of other fuel cell systems, at around 40 per cent, and they also take longer to warm up than PEM cells. They have been used to power buses and, following considerable research have been successfully developed for stationary applications. For example, units with outputs ranging from 0.2-20MW are providing power to hospitals, schools and small power stations around the world.

Molten Carbonate Fuel Cells (MCFC)
MCFCs use either molten lithium potassium or lithium sodium carbonate salts as the electrolyte which, when heated to a temperature of around 650°C, melt and generate carbonate ions. These flow from the cathode to the anode, where they combine with hydrogen to form water, carbon dioxide and electrons. The electrons are routed through an external circuit back to the cathode, generating power on the way. The high operation temperature means they are able to internally reform hydrocarbons, such as natural gas and petroleum, and can generate hydrogen within the fuel cell structure. At these temperatures there is no problem with carbon monoxide poisoning of the catalyst and platinum catalysts can be substituted for less expensive nickel ones. The excess heat generated can also be harnessed and used in combined heat and power plants. MCFCs can work at up to 60 per cent efficiency and this could rise to 80 per cent if the waste heat is used.

However, the high temperature means the cells take time to reach operating conditions, making them unsuitable for transport applications. Current demonstration plants have produced up to 2MW but designs up to 50 and 100MW capacity are being considered.

Solid Oxide Fuel Cells (SOFCs)
SOFCs have a solid ceramic electrolyte, such as zirconium oxide stabilized with yttrium or scandium oxide. These materials can operate at temperatures between 600°C and 1000°C and can be operated with fuels containing carbon monoxide as it will readily electrochemically oxidize. Similarly to MCFCs, SOFCs can reform hydrocarbon fuels internally and have significant advantages in efficiency and simplicity because they do not need an external reformer. They can use bio gases, petroleum or natural gas directly as the source of fuel.

SOFCs also show the highest tolerance to sulphur contamination of all the technologies developed to date. The solid electrolyte means these cells are more stable than MCFCs but the construction materials needed to contain the high temperatures generated tend to be more expensive. SOFCs can have high efficiencies of up to 60 per cent and are used for decentralised combined heat and power plants for industrial and domestic use.

Testing SOFCs
A key factor in the development of SOFCs is the electrochemical characterization of cells. There is no standard way of testing and many companies have created their own in-house systems. In each, test engineers measure set parameters, such as current density and voltage output, providing valuable feedback to the designers who can introduce new ideas to improve the performance of the SOFCs and further develop the technology.

One such method locates the fuel cell in a ceramic housing, before placing it in a furnace and loading the cell with an electrochemical interface. Different resistance contributions can then be detected which represent the quality of a cell. Figure 1 shows the ceramic housing assembly for such a test

Morgan Technical Ceramics has been working with Siemens to develop and supply the ceramic housing and fixtures used in their in-house SOFC testing. The ceramic housing must have a number of mechanical, chemical and electrical properties in order to withstand the harsh testing environment. Precise parts, to high tolerances, must also be able to be manufactured repeatedly.

SOFCs operate at high temperatures and the test housing must, therefore, be able to withstand temperatures of 1000°C for a long period of up to 10 hours.

The material must be highly electrically isolating. This is to prevent short circuits in the test which would distort the output voltage being measured and the subsequent test results.

Achieving a tight sealing of the housing is vital. If there are any gaps in the housing fuel can mix with the air at the edge of the cell and react. This will affect the voltage output and the results of the test. The material must have flat and straight surface to ensure tight sealing.

It is important for the material of the housing to have a similar coefficient of thermal expansion (CTE) to the cell itself. This means that the assembly expands at the same rate in the furnace and no cracking occurs. If cracks appear then the fuel and air will mix, jeopardising the results.

The material of choice, that meets all the above, is Morgan Technical Ceramics’ AL995 Alumina. Materials such as Teflon and Al2O3/MgO spinell have previously been tried for housings, but don’t offer the same high quality and combination of good mechanical and electrical properties as Alumina.

Next steps
With the growing demand for clean forms of energy across the globe, significant investment is needed in new generation systems that meet higher efficiency and environmental standards. Fuel cells offer a promising option for many applications, and the materials expertise and capability of companies such as Morgan Technical Ceramics is vital if large-scale power generation projects based on SOFCs are going to be developed.